There are two generally commonly used generator frame constructions. In one such construction, a base assembly is provided comprised primarily of a horizontal rectilinear I-beam structural assembly having an open center for receiving a stator frame core as well as pedestal supports at opposite ends of the open center for supporting the pedestals which, in turn, mount bearings for the generator rotor. The stator core is generally a separate unit which, after fabrication, including installation of laminations, windings and other essential stator core parts, is disposed in the opening in the I-beam base assembly. Subsequently, the rotor is installed, with the rotor pedestals disposed on the pedestal supports of the base. An outer housing is typically disposed over the stator core, rotor and pedestals, and rests on the base assembly. Trunnions are also typically mounted on the base assembly so that the entire generator can be lifted for transport to, and installation at, a working site. While this type of generator frame assembly is uniquely strong and stiff and thereby avoids sagging when the entire generator is lifted, there are substantial drawbacks to this design. Among the disadvantages are the very large weight of the generator frame with its I-beam base assembly. For example, the I-beam base assembly may comprise up to 10% of the overall weight of the generator unit. Additionally, in packaged units, there is always the danger of misalignment of the rotor in the unit as a result of sagging during transportation and/or installation. An I-beam base assembly, however, has the very great advantage of enabling pedestal mounting of the rotor bearings. This is particularly attractive because of the low cost of the I-beam supported pedestal supports and pedestals.
In another conventional generator frame design, end shields are supported by the generator frame. In these designs, a one-piece frame is provided with end shields. Because of the integral nature of the end shields with the frame, the end shield openings must be very large. Otherwise, it is difficult or impossible to provide a frame with sufficient size and strength while simultaneously providing sufficiently large openings to install the necessary elements within the generator frame, i.e., the laminations, windings, etc. Because of the necessity to provide very large openings, the end shields must be very structurally stiff and, hence, are necessarily formed at substantial expense. End shields also require substantial machining, are difficult to shim when locating the bearings, and generally require very tight tolerances during machining operations. While the typical I-beam base assembly and its associated disadvantages are eliminated by using end shields, low-cost pedestal supports or pedestals cannot be used with the end shields because the pedestal supports require substantial structural strength and support without limiting the size of the openings of the end shields. If so limited, interior parts of the generator cannot be moved through the openings for installation as part of the stator core.